Brazed-plate heat exchanger and air distillation device fitted with said exchanger

ABSTRACT

The heat exchanger can be used for the vaporization of a pressurized liquid and comprises at least one passage way (P) for a liquid extending in a first direction, being provided with exchange waves that are substantially parallel to the first direction and comprising at least one redistribution area associated with at least one lateral collector in an intermediate area of the length thereof, in addition to being provided with deviation waves extending in at least one second direction forming an angle with the first direction, whereby the redistribution area takes up the entire width of the passage way, in a crosswise position with respect to the first direction, whereby the deviation waves communicate with the exchange waves. The deviation waves are essentially as rigid as the exchange waves. The invention can be used with exchangers in pumped air-distillation devices.

The present invention relates to brazed-plate heat exchangers, of thetype comprising at least one fluid passage extending in a firstdirection, provided with exchange fins substantially parallel to thefirst direction and comprising, in an intermediate area of its length,at least one redistribution zone associated with at least one lateralcollector and provided with deflection fins extending along at least asecond direction forming an angle with the first direction, thisredistribution zone occupying the entire width of the passage, crosswiseto the first direction, the deflection fins all communicating withexchange fins.

The document “The standards of the brazed aluminum plate-fin heatexchanger manufacturer's association”, Alpema, first edition 1994,describes various heat-exchanger configurations of this type.

More particularly, although not exclusively, the present inventionrelates to heat exchangers for vaporizing pressurized liquids, inparticular pressurized cryogenic liquids.

Air separation apparatus of the “pump” type produces at least one gasfrom air (generally oxygen) in liquid form which is pressurized by apump before its vaporization, thus enabling the compression of the pureproducts in gaseous form by a compressor to be avoided, which isparticularly beneficial, both economically and technically, and saferwith oxygen. The pressurized liquid is warmed and vaporized in aheat-exchange line which generally consists of one or more exchangerbodies, typically of the plate type and made of brazed aluminum. At thepoint in the exchanger where the pressurized liquid stops vaporizing,there are abrupt variations in the exchange coefficient of this fluidresulting from flow fluctuations causing the product to pass from atwo-phase mixture state, with a high exchange coefficient, to a purelygaseous state, with a much lower exchange coefficient. This leads to anappreciable local variation in the temperature of the wall or plate andof the exchanger fins. When the temperature difference between the fluidbeing vaporized and the adjacent heat-transfer fluids is appreciable,possibly greatly exceeding 10° C., and typically between 10° C. and 30°C., the local temperature of the wall and of the fins may vary rapidlyand abruptly, typically by 5 to 15° C. in a repetitive manner, which isenough to cause thermal fatigue phenomena.

When these fluctuations occur in a homogeneous zone of the exchanger,the fatigue remains very limited. On the other hand, the fatigue becomesconsiderable when the fluid stops vaporizing opposite deflection finscontained in the heat-transfer fluid passages. This situationcorresponds, for example, to an outlet of partially cooled air intendedfor expansion in a turbine in order to keep the plant cold. This isbecause the thermal fatigue phenomena are aggravated by the thermal andmechanical heterogeneities of the structure, the distribution ordeflection fins being spaced further apart and thicker, and thereforestiffer, than the exchange fins, in order to provide a lower resistanceto the passage of fluids while providing the desired mechanicalstrength. Generally, the deflection fins have a pitch which is greaterthan the exchange fins in order to reduce the pressure drop of thedistribution zones, and consequently these deflection fins are thickerin order to withstand the pressure. Other unfavorable factors are thepresence of dead spots, and possibly of separation bars, in theredistribution zones.

Thus a tendency for the plates to fail in the boundary regions of thedeflection fins is observed.

These thermal fatigue phenomena are particularly difficult to prevent inthe intermediate redistribution zones where heat-transfer fluid isremoved from the container and/or is introduced into it. This isbecause, depending on the type of market for the plant, theend-of-vaporization point varies considerably in the adjacent passages,and it is not possible to counteract a large height in the heatingpassages in order to position the redistribution zones.

The object of the present invention is to provide a heat-exchangerstructure comprising at least one redistribution zone avoiding, inparticular, the drawbacks mentioned above.

To achieve this, the subject of the invention is an exchanger of theaforementioned type, characterized in that the deflection fins havesubstantially the same stiffness as the exchange fins.

With the arrangement according to the invention, it is thus possible toeliminate the dead spots in the redistribution zone, which preventshaving the surface of the exchanger follow only the temperature of thevaporized fluid, and provides good thermal and mechanical homogeneitybetween the redistribution zone and the exchange zones. The thermalfatigue phenomena are thus considerably reduced.

With such an arrangement, the distribution zone has a stiffnesscomparable to that of the rest of the passage, and heat exchange withthe fluids whose temperature changes the least and the most slowly isenhanced.

According to one characteristic of the invention, the deflection finshave substantially the same geometric proportions as the exchange fins,that is to say they have relatively little stiffness and provide a goodheat-exchange surface. In this context, the use of an internalseparation or sealing bar usually placed between the upstream anddownstream part of a redistribution zone is preferably precluded.

In contrast, according to one aspect of the invention, the deflectionfins and the exchange fins may be of different types (perforated andnonperforated, and/or serrated and nonserrated).

The object of the present invention is also to propose the applicationof such an exchanger to the vaporization of pressurized liquids,typically to the vaporization of pressurized cryogenic liquids.

For this reason, another subject of the invention is an air distillationapparatus producing at least one pressurized cryogenic liquid andequipped with at least one such exchanger serving to vaporize thisliquid.

Other features and advantages of the present invention will emerge fromthe following description of several embodiments, given by way ofillustration but in no way limiting, with reference to the appendeddrawings, in which:

FIG. 1 is a schematic view of one embodiment of a redistribution zone ofa heat exchanger according to the invention;

FIG. 2 is a schematic view of a redistribution zone of anotherembodiment of a heat exchanger according to the invention; and

FIG. 3 is a schematic view of yet another embodiment of a redistributionzone of a heat exchanger according to the invention.

In the following description, and in the drawings, identical or similarelements bear the same, but possibly subscripted, reference numbers. Inthe drawings, the exact geometric proportions are not respected, inorder to facilitate the reading thereof.

FIG. 1 shows, very schematically, a wall or plate 1 of a plate exchangermade of brazed metal separating two fluid passages, one P of which, asdrawn in cross section in the left part of FIG. 1, conveys a fluid whichtransforms by vaporization from a liquid state 2 to a gaseous state 3.At the interface zone 4 where the pressurized liquid phase stopsvaporizing, a redistribution zone 5 is shown in an adjacent passage P′shown in longitudinal section, from which the heat-transfer fluid,flowing normally along the wall 1 in the main flow direction of the heatexchanger (vertically downward in the figure) is deflected laterallytoward a collector 6 in order to be partially extracted from theexchanger (arrow E).

Conventionally, the wall 1 is fitted with relatively thin parallelexchange walls 7 of tight pitch (much tighter than shown in the figuresso as not to overburden them). According to the invention, in theembodiment of FIG. 1, the stream is deflected from and to the lateralcollector 6 by deflection fins 8 of the zone 5, having substantially thesame geometric characteristics (thickness, shape, density) as theexchange fins 7, but here extending orthogonally thereto. As can be seenin FIG. 1, the fins 8 are distributed over a prismatic volume with aV-shaped section extending progressively over the entire transversesection of the passage defined by the exchange fins 7, a mid fin 8 athus separating the passage into two zones, the one upper zone where thewhole of the stream F of heat-transfer fluid descending along the fins 7is deflected toward the collector 6, and the other lower zone where partof the heat-transfer fluid collected in the collector 6 is redistributedtoward the bottom of the passage P′ with exchange fins 7.

According to the invention, such an arrangement makes it possible toeliminate the internal sealing bars which constitute mechanicalinhomogeneities and which create dead spots, isolated from the passageof fluids and as a result sensitive only to the temperature variationson the other side of the wall.

Advantageously, in order to improve the distribution of the stream inthe deflection fins 8, a resistance to flow or pressure drop, typicallyconsisting of a short section of exchange fins 10 arranged in the “hardway”, that is to say orthogonally to the stream F and with baffle-typeor serrated-type offset openings, is placed upstream and/or downstreamof the deflection fins, as shown.

FIG. 2 shows an embodiment where the deflected flow E is preselected bydeflecting a predetermined part of the incoming stream F, part of whichpasses, with redistribution, directly through the redistribution zone 5.

Here, the deflection fins 8 make, with the exchange fins 7, an anglegreater than that of the diagonal of the distribution surface, the headof which is defined by the collector 6. One (8 b) of these fins, whichis connected to an exchange fin 7, defines the separation between thestream deflected in the collector 6 and the stream passing directlythrough the redistribution zone 5. The latter therefore has a righttrapezoidal section, with its apex located on the edge of the passageopposite the collector 6 and its upwardly directed oblique side.

FIG. 3 shows an embodiment similar to that of FIG. 2, but this time withdeflection fins 8 parallel to the diagonal of the distribution surface,which here is marked by a mid deflection fin 8 a extending transverselyfrom edge to edge of the main passage. Thus, the zone 5 has arectangular section and, in a manner similar to the embodiment of FIG.1, all the incoming stream F is deflected toward the right collector 6d, an external line 9 sending part of the stream extracted from theright collector 6 d back toward a left collector 6 g feeding half of thefin pattern 8 which communicates with the exchange fins 7 located belowthe redistribution zone 5. It will be noted, here again, according toone aspect of the invention, that the redistribution zone 5 iscompletely free of any internal sealing bar, a slight unwanted internalleak between the regions located on either side of the diagonal fin 8 anot causing any problems.

In each embodiment, the redistribution zone 5 opens out laterally overthe entire height of the or each associated collector.

In all cases, according to one aspect of the invention, the deflectionfins 8, whether or not of the same type as the exchange fins, aremorphologically similar to the latter, with equal or similar pitches (to±10%, ±20% or ±30%) and also of equal or similar thicknesses (to ±10%,±20% or ±30%), typically between 0.15 and 0.5 mm, and made of aluminum.Preferably, the same applies to the fins 10 for creating pressure loss,which may also be provided in the case of FIGS. 2 and 3, as indicated bybroken lines.

As mentioned above, such exchanger-distributor architectures, typicallyproduced by the assembly of brazed subassemblies and made of aluminumor, at least partly, of stainless steel, have a preferred application incryogenic air-distillation apparatus producing, in particular, liquidoxygen pressurized by a pump before its vaporization, under pressure insaid exchangers, by heat exchange in particular with pressurized air.

What is claimed is:
 1. A brazed-plate heat exchanger, of the typecomprising at least one fluid passage (P′) extending in a firstdirection, provided with exchange fins (7) substantially parallel to thefirst direction and comprising, in an intermediate area of its length,at least one redistribution zone (5) associated with at least onelateral collector (6) and provided with deflection fins (8) extendingalong at least a second direction forming an angle with the firstdirection, this redistribution zone occupying the entire width of thepassage, crosswise to the first direction, the deflection fins (8) allcommunicating with exchange fins (7), wherein the deflection fins (8)have substantially the same stiffness as the exchange fins (7) and theexchange fins (7) are associated with a structure (10) generatingpressure loss located upstream and/or downstream of the redistributionzone (5).
 2. The exchanger as claimed in claim 1, characterized in thatthe deflection fins (8) are morphologically similar to the exchange fins(7).
 3. The exchanger as claimed in claim 2, characterized in that thethicknesses of the deflection fins (8) and of the exchange fins (7) areequal.
 4. The exchanger as claimed in claim 2, characterized in that thethicknesses of the deflection fins (8) and of the exchange fins (7)differ from each other by at the most 30%.
 5. The exchanger as claimedin claim 2, characterized in that the thicknesses of the deflection fins(8) and of the exchange fins (7) differ from each other by at the most20%.
 6. The exchanger as claimed in claim 2, characterized in that thethicknesses of the deflection fins (8) and of the exchange fins (7)differ from each other by at the most 10%.
 7. The exchanger as claimedin claim 2, characterized in that the pitches of the deflection fins (8)and of the exchange fins (7) are equal.
 8. The exchanger as claimed inclaim 2, characterized in that the pitches of the deflection fins (8)and of the exchange fins (7) differ from each other by at the most 30%.9. The exchanger as claimed in claim 2, characterized in that thepitches of the deflection fins (8) and of the exchange fins (7) differfrom each other by at the most 20%.
 10. The exchanger as claimed inclaim 2, characterized in that the pitches of the deflection fins (8)and of the exchange fins (7) differ from each other by at the most 10%.11. The exchanger as claimed in claim 1, characterized in that thedeflection fins (8) have substantially the same geometriccharacteristics as the exchange fins (7).
 12. The exchanger as claimedin claim 1, characterized in that the deflection fins (8) are of thesame type as the exchange fins (7).
 13. The exchanger as claimed inclaim 1, characterized in that the redistribution zone (5) is free ofany internal sealing bar.
 14. The exchanger as claimed in claim 1,characterized in that the deflection fins (8) are orthogonal to thefirst direction.
 15. The exchanger as claimed in claim 1, characterizedin that one deflection fin (8 a) extends from one lateral edge to theother lateral edge of the distribution zone.
 16. The exchanger asclaimed in claim 1, characterized in that the redistribution zone (5)has a V-shaped section extending progressively over the entiretransverse section of the passage.
 17. The exchanger as claimed in claim1, characterized in that the structure (10) generating pressure lossconsists of fins of the hard-way type having a stiffness comparable tothat of the exchange fins (7).
 18. The exchanger as claimed in claim 17,characterized in that the fins (10) of the hard-way type aremorphologically similar to the exchange fins (7) with similar pitches to±30% and similar thicknesses to ±30%.
 19. The exchanger as claimed inclaim 18, characterized in that the fins (10) of the hard-way type areof the same type as the exchange fins (7).
 20. The exchanger as claimedin claim 1, characterized in that said passage (P′) is a heat-transferfluid passage adjacent to a liquid vaporization passage (P′).
 21. Theexchanger as claimed in claim 1, characterized in that it consists of anassembly of brazed subassemblies.
 22. The application of a heatexchanger as claimed in claim 1 to the vaporization of pressurizedcryogenic liquid.
 23. Air distillation apparatus producing at least onepressurized cryogenic liquid and equipped with at least one heatexchanger according to claim 1 serving to vaporize this liquid.